Loss measurement in two-way electrical transmission systems



Jan. 12, 1954 A. L. BONNER S MEASUREMEN LOS T IN TWO-WAY ELECTRICALTRANSMISSION SYSTEMS 4 Sheets-Sheet l Filed June 20, 1950 V -iwlvrl TIA-A :j

A. L. BONNER LOSS MEASUREMENT IN TWO- 4 Sheets-Sheet 2 Filed June 20,1950 I. m. ot Em R n F.. Wfww NW l w8 A WL AZ V. B

Jan. 12, 1954 A. L. BONNER LOSS MEASUREMENT IN TWO-WAY ELECTRICALTRANSMISSION SYSTEMS 4 Sheets-Sheet 3 Filed June 20, 1950 2,666,099RICAL a4 sheets-sheet 4 A. L. BONNER TRANSMISSION SYSTEMS Jan. 12, 1954.6

LOSS MEASUREMENT IN TWO-WAY ELECT Filed June 20, 1950 Patented Jan. 12,1954 LOSS MEASUREMENT IN TWO-WAY ELEC- TRICAL TRANSMISSION SYSTEMSArthur L. Bonner, Chatham, N. J., assigner to Bell TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication June 20, 1950, Serial No. 169,165

13 Claims. 1

This invention relates in general to electrical transmission systems andmore particularly to methods and apparatus applicable in such systems tothe measurement of transmission loss in both directions through atwo-way toll transmission circuit.

To maintain toll telephone circuits in proper operating condition it isnecessary to measure circuit transmission loss accuratley, frequentlyand with minimum interruption of service. The loss must be measured inboth directions of transmission, for the loss in one direction willgenerally not be the same as that in the opposite direction. This may beunderstood on considering the fact that between terminal switchingstations, where the circuit appears as a two-way two-wire circuit, thecircuit may take the iorm, for example, of a four-wire system, carrieror non-carrier, with separate repeatered paths for each direction oftransmission. Present practice requires the cooperative effort ofattendants at two terminal stations, which has the disadvantage oflimiting the opportunities for making measurements and of increasing thelikelihood of error. Further difficulties are encountered where one ofthe terminal stations, although provided with automatic switchingequipment, has no test attendant.

It is, therefore, a principal object of the present invention tosimplify the measurement of the loss in both directions between a firstand second terminal station; and more `specifically to provide for suchmeasurement with a minimum number of attendants. i

In accordance with the present invention the two-way measurement of lossof a signal transmission circuit extending between a first terminalstation A and a second terminal station B provided with automaticswitching equipment is carried out in two substantially concurrentsteps. One step is to transmit from, say, station A dial pulses, orother signals, that are operative on the automatic switching equipmentat station B to cause a tone of predetermined fixed strength to beapplied at B to the circuit under test. The strength of this tone asreceived at station A is measured, and the change in strength over thestrength of the tone applied at B is taken as a measure of thetransmission loss from B to A. The other step involves dialing at A toeiect connection of special test equipment to the circuit at B,transmission of a tone of predetermined xed strength over the circuitfrom A to B, the return ofa tone from B the strength of which isautomatically adjusted in predetermined relation to the strength of thetone received at B,

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and measurement of the strength of the returned tone at A. Thelast-mentioned measurement enables determination of the sum of therespective losses in the two directions of transmission (which may becalled the round-trip 1oss), so that by subtraction of the measured B toA loss, the loss in the A to B direction can be determined. Inasmuch asthe nist-mentioned step conforms with well-known practice thisspecification will be devoted largely to a description of thesecond-mentioned step, i. e., the measurement of round-trip loss.

in accordance with one embodiment of the invention, a tone of givenfrequency, say 1,000 cycles, having a fraction of a second duration, istransmitted at predetermined xed strength from station A to station Bwhere the strength of the received tone is used to adjust the loss of avariolosser circuit to correspond to the A to B loss of the circuit.After the sending tone has been removed rom station A, the vario-lossersetting is maintained for a short period by a holding circuit. Theadjusted vario-losser is then used to control the magnitude of anothertest tone of the same frequency and of slightly longer duration whichistransmitted from station B back to the station A where it is measuredby a transmission measuring set which has been substituted for the testtone source at station A. Inasmuch as the power level of the tonetransmitted from station B is below that which was originallytransmitted from the station A by the amount of the A to B loss of thecircuit, assuming that the generated tones at both stations are of thesame predetermined xed strengths, the receiving equipment at station Acan be calibrated to indicate the round-trip loss between stations A andB in terms of the received power level. Assuming that the B to A losshas been determined by means of a tone dialed up at station B, andtransmitted from B to A, the A to B loss may easily be computed.

In accordance with a second embodiment of the invention, the round-triploss is measured by transmitting a sustained tone oi given frequencyfrom station A which is frequency shifted at station B before beingtransmitted back again to station A. Assuming that a b-cycle test toneis transmitted from station A, it is converted at station B to aLOGO-cycle tone and again returned to station A. The conversionapparatus at station B is designed to have a zero loss; hence, thediierence between the transmitted and received 4powers at station Arepresents the roundtrip loss of the circuit.

The invention will be better understood from a detailed study of thespecification hereinafter with specific reference to the attacheddrawings in which:

Fig. 1 shows a typical circuit adaptable for testing in accordance withthe present invention;

Fig. 2 indicates the contemplated relationship of the test apparatus ofthe present invention to conventional automatic telephone switchingequipment and the connecting wire channels between two toll telephoneswitching centers.

Fig. 3 shows an embodiment of the invention utilizing a tone of shortduration transmitted from station A which is received at station B andutilized to control the amplitude of second short tone of the samefrequency transmitted back to station A; 1

Fig. 4A shows a detailed schematic of the circuit arrangement at stationB in the system of Fig. 3

Fig. 4B shows details of a typical delay circuit represented by blockdiagram 68 in the circuit of Fig. 4A; and

Fig. 5 shows a second embodiment of the invention in which the frequencyof a sustained tone transmitted from station A is converted to anotherfrequency at station B and returned again to station A.

Referring now to Fig.v l of the drawings, there is indicated a typicaltoll circuit of the type to which application of the present inventionis contemplated. The circuits'which serve to interconnect a pair ofswitching centers represented by a central onice A and a toll tandemoilice B comprise two-wire trunks at the terminals, which in generalfeed through some type of interconnecting hybrid junctions to four-wirerepeatered circuits which provide for separate transmission in the twodirections. The interconnecting toll trunk 'lines between the twoterminal sections may take various forms, including carrier andnon-carrier circuits, some comprising four-wire'circuits providingseparate transmission in the two directions, and others comprisingtwo-wire circuits in which the principal transmission path differencesin the two directions occur in passing through the v.

' repeaters.

Referring in detail to Fig. 2 of the drawings, the central oice A servesas the terminating point for a plurality of subscribersV lines L1;whereas, the

toll tandem otce B serves as the terminating point for a plurality ofdifferent subscribers lines L2. A plurality of toll trunk lines La areprovided for completing the subscribers circuits between points A and Bthrough the mechanisms of the automatic stepping selectors S1 and S2,which may z be assumed to be any of the types well known in the artwhich are ordinarily associated with telephone switching.

The testingl equipment in accordance with the present invention can beapplied to numerous different types oi telephone systems such asdescribed with reference to Fig. 1. For the purposes of illustration,the trunk lines L3 will be assumed to be a repeatered four-wire circuitwhich feeds into two-wire circuits at the terminals.

It is contemplated that the testing equipment in accordance with thepresent invention will be plugged into the terminal installations atstations A and B as indicated in Fig. 2 in such a manner as to makeconnections through the automatic switching equipments S1 and S2 vfortesting the trunk lines L3. At station A the transmission testingequipment TT1 may comprise a l-milliwatt tone source T1, a dial D', anda transmission measuring set TM. In accordance with one emselector S2,through the plug and jack J3.

4 bodiment of the invention, each of these elements is connected to adifferent terminal of a triple throw switch Ss, the armature of which isconnected through the plug and jack J1 to one of the steps of theautomatic switching selectors Si. At station B, transmission testingequipment TT2, corresponding to TT1 at station A, is connected to one ofthe steps on the automatic switching The transmission testy equipmentTTz functions to receive the tone transmitted from station A to VstationB, and transmit a tone from station B to station A having the same powerlevel as the tone received; at station B. This tone is then received atstation A and measured by the transmission measuring set TM, which iscalibrated to read the round-trip loss between A and B. The details ofthe test equipment TT2 will be described in detail hereinafter withreference to Figs. 3, 4A, and 5 which show two different embodiments ofthe invention.

Another step of switching selector S2 at station B is connected throughthe plug and jack J2 to a circuit which includes the dial pulse receiverDR responsive to tone transmitted from the dial D at station A. Thisreceiver DR is connected to initiate the operation of a second tonegenerator T3. Thus, in a manner well known in the art, when the dial Dis connected to transmit dial signal from station A, the pulse. isreceived by the dial pulse receiver DR, assuming `the jack `and plug J2is connected at station B. This in turn actuates the dial tone generatorT3 to transmit a loss in accordance with the present invention,v

which may be applied for testing trunk lines between two telephoneswitching centersin a manner described withV reference to Fig. 2. Inaccordance with this embodiment of the invention,vthe

Y test signal is` a tone of short duration traveling first in onedirection and then in the other. ,As in the foregoing description, A andB will-'be used to designate typical central and toll-tandem switchingcenters.

Referring to Fig. 3 assume that a two-way transmission circuit 9connecting these two stations is to be tested. For simplifying thediscussion that follows, the telephone switching cir-.

cuits are assumed to be included in the two-way circuit 9, only theschematic of the testing circuits being indicated as connectable intothe telephone circuits through the jacks J1 and J3 at the respectiveterminals. At station A, a generator I of a spurt of tone having adurationof a fraction of a second, and a frequency which in the presentillustrative embodiment will be assumed to be 1000 cycles, is connectedthroughv a three-terminal switch 3 corresponding to switch S3 in Fig. 2.With the switch in position 3cr, the generator I is connected across theline 9; and with the switch in position 3b, the transmission measuringset 'l is substituted for generator i. Transmissionmeasuring set l maytake the form of any of the types of loss measuring equipment well knowninthe art. Accordingly when the switch 3 is in position 3a, the line 9is conditioned to receive the LOGO-cycle tonefromthe generator I; andwhen the switch 3 is in position 3b the transmission measuring set 1 isconnected to receive output currents from the line 9.

At the other terminal station, B, equipment is connected for receivingthe single frequency spurt of tone generated at station A and forcontrolling the amplitude of a slightly longer spurt of tone of the samefrequency which is transmitted in the opposite direction from station Bto station A.

The hybrid coil II comprising conjugate pairs of arms lla, I Ib, andllc, IId serves to connect the transmitting and receiving branches ofthe circuit to the line 9 at station B. The receiving' and holdingequipment I3 which is connected to receive input signals through theterminals ila of the hybrid coil II, operates through its Variouscomponents to control the operation of the vario-losser and sendingrelay circuit I5, which in turn control the amplitude and duration,respectively of a second 1,000cycle tone which is transmitted from thetone generator I1 and is impressed on the line 9 through the terminalsIIb, IId of the hybrid coil Il for transmission in a reverse directionfrom station B to station A.

The circuit connections of the equipment of station B will now bedescribed in detail with reference to Figs. 4A and 4B of the drawings,which show the components thereof in detailed schematic. The operationof these circuits will be described hereinafter. Connected across theinput terminals IIa of the hybrid junction I I, is a control circuit 2Iwhich comprises a conventional amplifier 3| characterized by an inputimpedance of approximately 600 ohms and a low impedance output, andwhich is designed to have a gain of approximately 30 decibels. Theoutput terminals of the amplier 3l are connected through a rectifyingcircuit 33 which may cornprise, for example, a copper oxide varistorhaving four arms connected in bridge relation, each of which armsincludes four three-sixteenths of an inch copper oxide discs. The outputof the rectifying circuit 33 is impressed across the 25,000-ohm resistor35, across which it develops a potential which is impressed on the inputterminals of the holding circuit 25 through the contacts 31a of therelay 31 when relay 31 has been operated by relay circuit 23 and relay65, as will be described hereinafter.

In parallel with the control circuit 2l across the terminals IIa of thehybrid junction II, is connected the relay amplifier-rectifier circuit23. This includes the input transformer 39, which preferably has animpedance ratio of 600 to 700,000, and across whose secondary isconnected the 700,000-ohm potential divider 4I. An ad justable slider incontact with the potential dvider 4I is connected to the control grid ofthe power pentode 41, the rectied output current from which is utilizedto drive the relay 95. The pentode 41 includes a cathode connected toground through the E500-ohm resistance element 5I, a suppressor griddirectly connected to its cathode, and a, plate which is coupled throughthe output transformer 55 and the Vrectifying circuit 63 to theenergizing windings of the relay 65. The transformer 55, whichpreferably has an impedance ratio of 20,000 to 50, has one of theterminals of its primary coil connected to the plate of the pentode 41,and the other terminal connected to 13G-volt plate battery 54 and to itsscreen grid through the 33,000-ohm resistor 59, a path to ground beingprovided from the screen grid through the 0.5-microfarad condenser El.The rectier circuit 63 may, for example, comthe source of potential 40.

6 prise a copper oi'ride varistor which has four arms in bridgeconnection, each arm comprising two three-quarter inch discs.

The relay 65, bias for which is furnished by the potential source 66operating through the current limiting resistor 62 is preferablypolarized in order that it may operate on the low power furnished fromthe amplifier pentode 41 and the associated output transformer 55. Therelay amplifer-rectiiier circuit 23, as described, is designed tofurnish a current of about ten mils into the 20D-ohm relay 85, assumingthat the power received at the input terminals to the oiiice B is about22 decibels below one milliwatt. If it is desired to measure wire lossesgreater than 22 decibels, a non-sensitive form of the relay 65 may beused. Relays are available which will operate on a current of about twomils corresponding to an input power of about 36 decibels below onemilliwatt.

The relay 65 is designed so that when operated it makes contact 59awhich completes a circuit including the energizing coil of the relay 31,the normally closed contact 39a of the reiay 39, and The relay 31 havingthe normally closed contact 3117, and the normally opened contact 31a,is a general utility relay which has no specialized requirement. TheContact 31a of the relay 31 is connected to impress the rectifieddirect-current potential drop across the resistor 35, across the inputterminals of the holding circuit 25. The contact 31b of the relay 31connects ground to a parallel circuit, one branch of which includes therelay 69 and its energizing source 19 and the other branch of whichpasses through the delay circuit 98 to energize the slow-operating relay1I, through the source 12. The slow-operate relay 1I is preferably of atype constructed with a copper sleeve, providing a release time which isslower than that of the relay 69.

A form which the delay circuit 28 may assume is shown in detail in theillustrative diagram of Fig. 4B. This includes the relay 92, energizedby the source 93, the relay 9d energized by the source 95, the relay 9&5energized by the source 91, and the relay 98 energized by the source 99.It will be noted that the path between the input and output leads 88 and99 of the delay circuit 68 passes through the normally closed contact99h or the relay 95, and the normally opened contacts 981) of the relay90, so that the relay 96 must be released and the relay 98 must beoperated in order that the path be closed. When ground, through contact31h, is applied to the input terminal of the delay circuit 98, therelays 92, 94, and 93 are operated in succession. The series pathbetween the inputand output leads B3 and 90 is then opened at thecontacts 9612 of the relay 96 and closed at the contacts 98h of therelay 98. In addition, the relay 98 is locked up under control of itsown contacts 98a and contacts 311) of relay 31 and the operating' pathof the slow-release relays 92, 9d, and 99 is opened at the uppermostcontacts of relay 99. The relays 92, 94 and 99 then successivelyrelease, the release of the relay 99, through contacts 9619 completingthe path between the input and the output leads to 88 and 99 of thedelay circuit 6B. The time delay obtained by the operation and releaseof these relays is approximately 1.8 seconds, thus providing a delaywhich is sufficient for either visual reading of a type of metercommonly used in transmission measuring sets, or for accanto thevmechanical setting of a lock-up type of meter in which the meter needleis locked by a relay.

As has been explained, the purpose of the control equipment at station Bis t receive power from the station A, to store information regardingthe level of the received power, to set the gain of the vario-lossercircuit 2i accordingly, and to4 transmit back to station A a toneindicating the level of tone received from station A when the tone fromthe station A is removed. The storing of information and the setting ofcircuit 21 are accomplished by means of the control circuit 2i, and theamplifier-rectier circuit 23, operating in conjunction with the holdingcircuit 25, which functions to maintain the loss ofthe vario-losser 2lfor a few seconds after the control tone from station A is removed,whereby the control current 'flowing in the vario-losser circuit may bemaintained at a desired value. The holding circuit 25 includes thecl-microfarad condenser 'i3 across the high impedance input terminals tothe control grid of pentode 15. An alternative biasing connection forthe control grid passes through the normally operated contacts 1lb ofthe relay 7i to ground through the LOCO-ohm resistor 1li. Thesuppressor, plate and screen grids of the tube 'E5 are connectedtogether to the energizing G-volt potential source The 2,000-ohm cathoderesistor 'it is connected to the center tap of the transformer it in thevario-losser circuit 2l.

The cathode-follower circuit described above is designed to give acathode current which is very nearly proportional to the voltageimpressed across the grid by the condenser i3. The elements of thecontrol circuit 2l, and the ampliner and rectifier circuits 23 have beendesigned to produce enough voltage across the condenser 1.3 to permitthe use oi the high cathode resistance 71S, whereby a substantiallylinear voltage current relationship is obtained in the cathode followercircuit 25 including the pentode i5.

Tone for return transmission from station B to station A is derived froma conventional .LGSS- cycle generator ll which is connected undercontrol of the normally released contacts Stb and 3&0, of the generalutility sending relay 3S, to the input terminals of the vario-lossercircuit 21' through the terminals of the balanced step-up transformerhaving a preferred impedance ratio of 609-200. Relay 3% energized by thebattery Sii through a circuit which includes the normally closed contactita of the relay Se, and the normally opened contact lia of theslow-operate relay i i.

The vario-losser circuit 2- is of the general type described in anarticle entitled Vario Losser Circuits by VLIR; Bennett and S. Debapublished in the Transactions of the American Institute of ElectricalEngineers, volume 60, pages 17 through 32, January 194i. It maycomprise, for example, four copper oxide varistors, one pair connectedin series between the upper terminals and the other pair connected inseries between the lower terminals of secondary coil of the inputtransformer Si? the primary coil of the substantially similar outputtransformer St. Each one of the varistors of the circuit di maycomprise, for example, four three-sixteenth of an inch copper discs. Asdescribed above the center tap of the input transformer il@ is connectedthrough lead 'il' to the control current from the holding circuit 25.

Connections from the output translormer B are made through the amplifier@I to terminals s Hb of the hybridr coil H for thefpurpose of comeYpensating the three-decibel lossV sustained through the hybrid coil l l,and the loss of approximately 13 decibels sustained through thevario-losser circuit 2l' when the input power impressed on terminals Hdof the hybrid coil is zero decibels below one milliwatt. This assumesthat the-line S of Fig. 3 has Zero loss.

Operation of the circuit of Fig. 3 and Fig. 4A, indicating the detailsof station B, which have been structurally described in the foregoingvparagraphs, is as follows. When a LOGO-cycle tone from the generator lat oflice A is applied through the contact te of the switch 3 to aselected test line 9, the tone received at the oflice B passes throughthe hybrid junction l l producing a voltage across its terminals licwhich is rectified by the control circuit 2i to produce a voltage dropacross the resistor 35. Simultaneously the tone received at office B isalso applied through the terminals im of the hybrid coil U to the relaycircuit amplifier-rectifier 253, the rectified output from whichoperates a sensitive relay @5 by overcoming the bias. The relay 65 in .Yturn operates relay 3i to engage its contact 31a,

thereby impressing the voltage drop developed across resistance 35 andhence across the plates of condenser 13, to the control grid in thecathode-follower pentode 'i5 of the voltage storing holding circuit 25.Y

Normally, relays et, 98 and l'l are in an ener, giaed condition, so thatthe operation of the re.- lay t? opening its contacts 31h to groundcauses these relays to release. Since the relay 'Il has a copper sleeve,it will accordingly release more slowly than relay 5S, and hence thesending relay 3Q will not be operated.

when sufficient time has elapsed to charge the condenser i3, thegenerator l producing the Lecc-cycle tone at the cnice A may be replacedby a transmission measuring set 1, either by manual operation of theswitch 3 to engage its contact 3b or by some automatic means. A typicalcharging time for the condenser 13 is about 0.3 second.

After the generator i has been replaced by the transmission measuringset at station A, the following operations occur at station 13. Therelays and 3l release, thereby operating relay 69 which, in turn,operates the sending relay 39 through the contacts 69a of relay Sal andlla of the slow-operate relay H, operation of which is delayed by thepresence 0f the delay circuit 68. The sending relay 3Q will remainoperated until the slow-operate relay il operates after an interval ofabout 1.8 seconds.

Simultaneously, with the impression of the 1,006 cycle tone on the relaycircuit amplifierrecti'ier 23, the LOGS-cycle tone from station A isalso amplified and rectified in control circuit 2i, impressing apotential across the resistor 35, which charges up the condenser 'F3connected to the control grid of cathode follower l5. The output fromthe cathode follower passes to the variolosser 2l' through control leadll, and functions in the manner described by Bennett and Doba in theaforementioned article in the Transactions of the American `Institute orElectrical Engineers to adjust the loss of the vario-losser 27.Accordingl the output-from the transformer 89 is'reduced from areference or calibrated value of about 13 decibels by an amountcorresponding t0 the loss of the line. For example, if the input powerto the control circuit amplier 3i is down by 5 decibles, from that ofthe generator at sta:

tion A, the power from the output terminals of the vario-losser circuit2l will accordingly be reduced 5 decibels. f

Meanwhile, during the period relay 39 is operated, a LOGO-cycle tone istransmitted by the generator I1 through the adjusted vario-lossercircuit 2l, the amplifier 9|, the hybrid coil I I and the line 9, whichwill be received and measured by the transmission measuring set 'I atoiiice A. When the sendingr relay 39 operated, it opened the operatingpath of the relay 31, by disengaging contact 39a before the 1,000-cycletone is applied through the closure of the contacts 30o and 39e. This isnecessary in order to prevent operation of the relay 31 by the generatorI'I. The operation of the slow-operate relay 'Il discharges thecondenser 13, and releases the sending relay 39, thus conditioning thecircuit for another measurement.

Fig. 5 of the drawings shows an alternative form of the invention inwhich the round-trip loss measurement is made by tones of two differentfrequencies. It is apparent that the system which is about to bedescribed can be applied for testing toll lines between switchingcenters in the manner indicated in Fig. 2. In accordance with thisembodiment, a test tone of given frequency is transmitted from thecontrol oice A to the distant office B, where it is converted to a toneof slightly different frequency, and returned to office A. Inasmuch asthe apparatus at station E is designed to have zero loss, the differencebetween the transmitted and received powers at oiiice A represents theround-trip loss of the circuit. For the purposes of the presentillustration, 900 cycles per second has been chosen as the frequency forthe tone transmitted from station A; and 1,000 cycles as the convertedfrequency of the tone returned to station A from station B.

At station A, the hybrid coil |536 serves to isolate the S200-cyclegenerator Illl which is connected to its terminals Ia, from thetransmission measuring set I0?, which is connected to the conjugateterminals |0617. A LOUD-cycle filter |54 is included in the path to themeasuring set |01,

which filter is preferably designed to have a 40- decibel discriminationat 900 cycles, in order that a high loss reading in the measuring setIQ? is not appreciably aiected by the transmitted energy. Such arequirement can easily be met by using two filters in tandem, of a typesuch as commonly used in voice frequency telegraph systems. The secondpair of conjugate terminals 206e, IIlId are respectively connected tothe test line |09 and a balancing network I 04.

The circuit at oflice B includes a 90o-cycle band-pass filter 124, whichfunctions to receive the tone transmitted from station A, a modulator|20, which functions to shift the frequency from S00 to l,000 cycles bymodulating the input signal with a L5100-cycle signal from the generator|22, a LUGG-cycle band-pass filter H4 the purpose of which is to reduceunwanted modulation products, and an amplifier |i 2 which compensatesfor the signal loss introduced by the circuit elements at station B. Thehybrid coil" II! serves to isolate the output of the amplifier I I2 fromthe input to the S300-cycle band-pass filter 24, respectively connectedto the terminals |IIa and IIIb. The second set of conjugate terminalsIIId and IiIc are respectively connected to the line |89 and balancingnetwork I I0.

Certain considerations which enter into the design of the circuit are'the reduction of nterf'erence "with the.'measu1"enient from` theunwanted modulation products, and further, avoidance of self-oscillationin the loop at station B.

The double balanced type of modulator indicated as elements |20, on Fig.5, which is coupled to the output terminals of the S900-cycle filter |24through the balanced transformer I IS, and to the input terminals of theLOGO-cycle lter II4 through the transformer III, is preferably of a typesuch that all of the unwanted modulation products in the frequency bandnear 1,000 cycles per second are attenuated at least 25 decibels.Accordingly, the filtering provided need only be designed to suppressthe upper sideband.

Taking into consideration the feedback around the loop at oiiice B, theSOO-cycle filter |24 should preferably have sufficient discrimination ata frequency of 1,000 cycles per second and the 1,000- cycle filter I|4should preferably have sufficient discrimination at 900 cycles persecond, to keep feedback energy of the measuring frequency fromaffecting the measurement. Desirable design criterions provideconditions such that the sum of the discriminations mentioned should beof the order of 20 decibels, and that the sum of the losses of thefilters |24 and IIli should be at least 10 decibels at all frequenciesin the pass band of the amplifier I I2, in order to insure an amplemargin for avoiding oscillation of the circuit.

Although the invention has been described with reference to certainspecific embodiments, it is apparent that the invention may be embodiedin other forms also.

What is claimed is:

1. In an electrical system including a two-way i' signal transmissionpath extending between a first terminal and a second terminal, themethod which comprises transmitting a first tone over the signaltransmission path from said first terminal to said second terminal,utilizing the received energy of said first tone at said second terminalto adjust the amplitude of a second tone initiated at said secondterminal, returning said second tone over the signal transmission pathto said first terminal, and measuring the amplitude of said second toneat said first terminal with reference to the transmitted amplitude ofsaid first tone.

2. In an electrical transmission system including a two-way path forsignal transmission, a first source of tone connected to said path atsaid first terminal, a second source of tone connected to said path atsaid second terminal, a receiver connected to said path at said secondterminal for receiving said first tone over said path from said firstterminal, a control device connected to said receiver and said secondsource at said second terminal for controlling the amplitude of saidsecond tone transmitted over said path to .said first terminal inaccordance with the received amplitude of said rst tone, a receiverconnected to transmitted amplitude of said first tone.

3. In an electrical transmission system including a two-Way path forsignal transmission, a source of Atone of first frequency connected tosaid path at said first terminal, means connected to said path at saidsecond terminal for receiving and shifting the frequency of the tonereceived from said first terminal to a second frequency, an amplifierconnected to said frequency shifting circuit adjusted to compensate forthe loss therethrough, means lconnected to Saidpathat said second`terminal for transmitting said :second frequency'tone to said firstterminal, and .ameasuring device connected to said path at said irstterminal for'measuring the amplitude of said second tone received atsaid first terminal With reerence to the transmitted amplitude of saidrst frequency tone.

4. In an electrical transmission system including a two-Way path forsignal transmission between a control .station and an auxiliary station,a generator of a first spurt of pilot tone for connection tosaid path atsaid 'controlstatiom means connected to said pathat said auxiliarystation for receiving and registering the amplitude of said first spurttone, a generator of asecond spurt tone connected to said path at saidauxiliary station, means connected .to the last said generator and undercontrol of said receiving and registering means for controlling theamplitude of said second spurt tone impressed on said path in accordancewith the received amplitude of saidrst spurt tone at said auxiliarystation, Vand a measuring device connectable to said pathat said controlstation for measuring 'the received amplitude of said second spurt toneWith reference to the transmitted amplitude of said iirst spurt tone.

5. Anelectrical transmission system in accordance with claim 4 in whichsaid rst and said second spurts of tone are of the same frequency.

6. A system for "measuring the loss of an electrical transmission line,said line connecting a control terminal and an auxiliary terminal,.saidsystem comprising at said .control terminal: a first generator .of agiven frequency pilot tone, a transmission measuring set, and switching-means for alternatively connecting Asaid nrst generator vor saidmeasuring set to'said line, at said auxiliary terminal: receiving andstoring means connected to said line for receiving the pilot tone fromsaid controlstation vand storing energy pro- Aportional to the amplitudeof the received signal, a rsecond generator of a given frequency pilottone, and a loss-adjusting circuit connected to `said receiving andstoring circuit, said generator and said line, said loss-adjustingvcircuit responsive to energy received from said storing circuit tovcontrol the output energy of the pilot signal vimpressed on said Ilineby said second generator for return transmission to said vcontrolstation.

'LA system for measuring round-trip loss .in

. an electrical transmission system in accordance 'with' claim 6 inwhich amplifying means having Ja Ygain 'suicient to compensate yfor thevloss through said loss-adjusting circuit is :interposed between saidloss-adjusting circuit and `connection to said lline.

8. A system for measuring the round-trip lloss of an electricaltransmission line, said line .connecting a .control .terminal .and .anauxiliary 4ter- `minaLsaid system comprising-fat said control .termural:a .'rst generator `of `a given :frequency '.tone, a transmission:measuring set, switching lm'e'ansiforalternativelyLeonnectingsaid iirstgenerator'or saidmeasuring -set to .said line, Aatxsaid auxiliaryterminal: ia .hybrid junction .having a pair `.of terminals in conjugaterelation, `and a third .terminal connected to said line, .receiving andstoringmeans connected .to .one ofthe teriminals .of said pair for.receiving lthe pilot tone from Vsaid control `station and storing.energypro- Sportioned .to the-amplitude of the received signal, vasecond vgenerator of a :pilot tone of said given frequency, aloss-adjusting circuit connected between said second' generator "andVthe "othmf :ter-- 1n'iinalsfor Fsai'dip'ar 'rand vl.responsive Lto.energyzreceived from said .holding `circuit to control :the utputenergy of the pilot tone'. impressed on said line by said secondgenerator.

9. A system for measuring the round-trip Vloss of .an electricaltransmission line, said line connectlng Va control station and anauxiliary 'station, said system comprising at said control .station: yagenerator of pilot tone of a rst frequency, a transmission measuringset, a hybrid junction having a rst and a second set of Vterminals inconjugate relationship which are respectively connected to saidgenerator of a .first frequency pilot tone and said transmissionmeasuring set, and a third set of terminals connected to said line, atsaid auxiliary station: Ia hybrid junction having a Vfirst set ofterminals connected to said line, and a second and a third set ofterminals in conjugate relation, a modulator having its input terminalsconnected to said line through a path `Which includes said second set ofterminals of .said hybrid junction, a generator connected .to impress amodulating frequenc-y on said modulator for converting the frequency of'said pilot tone rom said rst frequency to a second frequency, and`amplifying means connected to receive the modulated output energy fromsaid modulator, said amplifying means having again such that vitcompensates Vfor the net loss through said auxiliary station, and saidampliiier connected to said'line through a path which includes saidthird 'set of terminals of said hybrid junction.

li). A system for measuring the round-trip loss of an velectricaltransmission line, said line connecting 'a control station and anauxiliary station, said system comprising at said control station: ahybrid junction having a first 'anda second set of terminals inconjugate rel-ation, and a third set of terminals of said hybridjunction connected to said line, a generator of a pilot tone of afrequency connectedto said first terminals of said hybrid junction, atransmission measuring set, a lter having its output terminals connectedto the terminals of said ltransmission measuring set and its inputterminals connected to the second set of terminals of said hybridjunction, said lter having a pass frequency which corresponds to avsecond frequency of said pilot tone, at said auxiliary station: ahybrid junction having a rst set of terminals connected to said lineVand a second and a third set of terminals in conjugate relation, 'afilter having a pass frequency corresponding to the first frequency ofsaid pilot tone connected to 'said line through said second terminals, amodulator connected to receive input energy through said lter, agenerator connected to impress a modulating frequency on said modulatorfor .converting .the frequency-of Asaid pilottone to a second frequency,a lter connected to receive output energy from said modulator, .saidiilter .having a pass band equal Yto said second frequency of said pilottone, and amplifying means connected to receive the modulated outputenergy .through said filter, said amplifying means having gain such thatit com'- pensates Vfor `the Anet loss through said auxiliary stationincluding `said modulator, and said arnplifier connected to said linethrough la path Which'includes said third `set of terminals of saidhybrid junction.

ll. In a system including va two-Way toll telephone transmission circuitinterconnecting two geographically separated points A and B, themethodof Vtransmission measurement which comprises applying 'to saidcircuit at Al a first tone current of predetermined xed strength,receiving said tone current from said circuit at B, generating at B asecond tone current of predetermined Xed strength and applying saidsecond tone current to said circuit at B, modifying the strength of saidsecond tone current, as so ap plied, under the control of said receivediirst tone and to an extent uniquely determined by the strength of saidreceived first tone, selectively receiving said second tone current fromsaid circuit at A, and measuring the strength thereof.

12. In a system including a two-way toll telephone transmission circuitinterconnecting tvvo geographically separated points A and B, the methodof measuring transmission loss which comprises applying to vsaid circuitat A a rst test signal of predetermined fixed strength, receiving saidtest signal from said circuit at B, generating at B a second test signalof predetermined xed strength, applying said second test signal to saidcircuit at B, modifying the strength of said second test signal, as soapplied, under the control of said received iirst test signal and to anextent fixed by the strength of said received first test signal,selectively receiving said second test signal from said circuit at A,measuring the strength thereof, and measuring substantiallycontemporaneously the transmission loss of said circuit in the directionB to A, whereby the transmission loss of said circuit in the oppositedirection can be evaluated.

13. In a system including a two-Way toll telephone transmission circuitinterconnecting two geographically separated points A and B, the methodof transmission measurement which comprises applying to said circuit atA a rst tone current of predetermined xed strength, receiving said tonecurrent from said circuit at B, applying to the circuit at B a tonecurrent of a strength having a predetermined relation to the strength ofthe received first tone, selectively receiving said second tone currentfrom said circuit at A, and measuring the strength thereof.

ARTHUR L. BONNER.

References Cited in the le of this patent UNITED STATES PATENTS NumberName Date 2,214,130 Green et al. Sept. 10, 1940 2,281,508 LundstromApr.. 28, 1942 2,322,330 Vroom June 22, 1943 2,337,540 Burgess Dec. 28,1943

